CeO.sub.2 belongs to a fluorite structure type compound and is known to become a solid electrolyte material showing high oxygen ionic conduction, when a trivalent rare earth element (such as Y, Sm or Nd) is solid-solubilized at tetravalent Ce sites to introduce oxygen vacancies (H. Yahiro, Y. Baba, K. Eguchi and H. Arai, J. Electrochem. Soc., vol. 135, 2077-80 (1988), and T. Inoue, T. Setoguchi, K. Eguchi and H. Arai, Solid State Inonics. vol. 36, 71-75 (1989)).
In such a defective ceria type solid electrolyte, oxygen vacancies are formed by substituting and solid-solubilizing a trivalent rare earth element at tetravalent Ce sites. However, if the amount of the trivalent element for solid solubilization is increased in order to increase the quantity of oxygen vacancies, a C type rare earth compound tends to form, and the conductivity tends to decrease by the formation of such a compound, whereby it has been difficult to improve the oxygen ionic conductivity.
Further, the defective ceria type solid electrolyte has had a drawback that Ce.sup.4+ in CeO.sub.2 is readily reduced to Ce.sup.3+ in a reducing atmosphere, whereby not only the oxygen ionic conductivity but also electric conductivity will appear. If the electric conductivity becomes remarkably high, when the electrolyte is used as a cell material for a fuel cell, the power density tends to decrease. The susceptivity to reduction of Ce.sup.4+ under such a working condition of a fuel cell has been a serious problem which hinders practical application of the defective ceria type solid electrolyte.
The present invention has been made in view of such a problem, and it is an object of the present invention to provide a fluorite structure type ceria type solid electrolyte which has ionic conductivity and reduction resistance of Ce.sup.4+ substantially improved over the conventional electrolytes and which shows an excellent power density when used as a cell material for a fuel cell.